In custom refractive surgery on the eye, the analysis portion of the process is typically carried out prior to the surgery, for example, with a wavefront aberrometer. Subsequent to the wavefront measurement, the laser ablation portion is directed to follow a prescription generated by the analysis portion. These procedures are often separated by days.
It is desirable to align the aberrometer results with the surgical system so that the ablation is positioned properly. Aligning images of the eye with high precision is challenging. The pupil, which can be easy to find and align, varies in size over short intervals. Further, the position of the pupil within the iris changes depending upon whether it is contracting or dilating, making high-precision alignment of two eye images difficult with the use of the pupil.
The limbus remains fixed between two images, but presents a soft boundary that changes appearance with illumination. A difficult alignment component is achieving relative rotation between the two images, since the main alignment markers, the pupil and the limbus, are rotationally symmetric. It is known in the art to use artificial alignment marks placed on the eye to provide a reference. However, these marks have conflicting requirements, in that they be stable and easily seen for alignment, but easily removed after surgery. It would be highly inconvenient to retain these marks in place for the period between measurement and surgery.
Algorithms are known in the art for assisting in image alignment; however, the surgeon must confirm this alignment before proceeding with surgery. It would be beneficial to provide an image alignment process that verifies alignment and also permits adjustment of the determined alignment.